Primary pumps of fast neutron nuclear reactors which are generally cooled by a liquid metal such as sodium are disposed, in the case where the reactor is of the integrated type, inside the vessel of the reactor which also encloses the core immersed in the primary liquid sodium filling the vessel. These primary pumps comprise a generally cylindrical body disposed vertically and supported in its upper part by the slab of the reactor closing the vessel. The pump body communicates, in its lower part, with a discharge tank which generally has a spherical shape. This spherical tank is connected to two discharge pipes which open, at their end opposed to the end connected to the spherical tank, onto the bed supporting the core of the reactor.
The liquid sodium which is drawn in by the pump in the region of ports extending through the case of its vertical body is discharged into the spherical tank and distributed in the discharge pipes and then re-injected under the assemblies of the core of the reactor.
The discharge sphere is rigidly fixed to the plating which is a fixed structure of the reactor resting on the bottom of the vessel and carrying the bed which acts as a support for the assemblies of the core.
Upon rapid variations in the operating conditions of the reactor, accompanied by rapid and large-amplitude variations in the temperature of the coolant fluid, and in the case of an abnormal operation of the reactor owing to an incident, which is also accompanied by large temperature variations, the discharge sphere, which is rigidly connected to the plating, is subject to considerable stresses of thermal origin. Indeed, the sphere has a tendency to move under the effect of variations in expansions accompanying the temperature variations, but it is practically immobilized by its rigid connection to the plating. High stresses result in the materials constituting the discharge sphere and the elements connected thereto.